The orthogonal frequency division multiplexing (OFDM) modulation method (hereinafter referred to as OFDM method) and the quardrature amplitude modulation method (hereinafter referred to as QAM method) are adopted in the terrestrial digital broadcasting and the multimedia broadcasting. The modulation signal in the terrestrial digital broadcasting and the multimedia broadcasting is constructed by constituent unit periods and has average power and peak power of a signal which are considerably different.
When the linearity is ensured by supplying a power amplifier with such a fixed voltage as to be able to produce peak power in a power amplification system, the time that the peak power is produced is extremely short and as a result the power supply efficiency of the amplifier is reduced. As a technique for solving this problem, there is known an envelope tracking method power amplification system (EER) as described in a patent literature 4. When a power supply voltage of an amplifier is changed, the characteristics of the amplifier are also changed. When a class AB push-pull amplifier is adopted in order to improve the efficiency, the characteristics of the amplifier are also varied at small amplification and peak.
By the way, an MOS-FET makes conduction only by applying a voltage between a source and a gate thereof and accordingly a transient response thereof in the direction of turning-on is fast, although since the MOS-FET continues to be conductive until electrical charges are removed from the gate thereof, a transient response in the direction of turning-off is slow. Hence, asymmetrical distortion is made large in an up-and-down direction of waveform on a time axis and even in an up-and-down direction on a frequency axis. Moreover, the MOS-FET has a conductive resistance varied in temperature. Further, in an MOS-FET made of GaN, electrons are trapped in a gate electrode in proportion to an accumulation amount of an electric field on a gate by a voltage on a drain electrode, so that a conductive resistance thereof is deteriorated (refer to a non-patent literature 7). These variation and deterioration are named historical distortion or memory distortion generically.
As a conventional technique of pre-compensation of nonlinear distortion, specifically as an example of a technique of independent compensation of odd order distortion, the patent literature 1 may be referred to.
A high-frequency power amplifier has so-called remarkable memory effect in which a past signal is generally influenced with extension to a wide band of signal and distortion is increased, so that hysteretic characteristic and even symmetrical distortion are increased. Thus, a circuit scale for the pre-compensation of distortion is enlarged. Accordingly, a method of reducing the circuit scale of the pre-compensation of distortion by the memory effect of the high-frequency power amplifier is proposed in a non-patent literature 1.
A patent literature 2 discloses a pre-compensation technique of distortion using time difference of the even order.
A patent literature 3 discloses a pre-compensation technique of distortion using differentiation of amplitude and phase.
However, the techniques for compensating the even order distortion by the differentiation of amplitude and phase in the patent literatures 2 and 3 have a drawback that convergence of the pre-compensation for reducing the even order distortion of the memory effect requires time even if the even order distortion of the memory effect is changed.
Furthermore, a Cartesian loop transmitter feeds back an output signal of an amplifier to a baseband part and compares signals before and behind amplification with each other to make detection and correction of error, so that the linearity of the transmitter can be increased. However, input/output and wiring of a chip are contained in a path for the feedback and accordingly transmission of signals is delayed for that. Influence of this delay is increased in proportion to frequency and accordingly there arises a problem that the stability is deteriorated when the band is widened. Accordingly, this time, a route which does not pass through a frequency converter is added in addition to the feedback path in the prior art and only a high-frequency component which influences the stability passes through the route to thereby reduce the influence of delay (refer to non-patent literature 6). In the non-patent literature 6, a complicated analog feedback path is added, so that it is difficult to make application to large-power amplification.
Accordingly, in the wide-band OFDM, it is difficult to improve the efficiency by combining the EER as described in patent literature 4 in which a power supply voltage of an amplifier is varied to change even characteristics of the amplifier with the pre-compensation technique of distortion as described in patent literatures 1 to 5 in which the convergence requires time.
Hence, as described in the patent literature 5, RF input and RF feedback are subjected to FFT and distortion coefficients of AM/PM conversion distortion, spectral re-growth distortion and memory effect distortion are calculated. The technique that RF input power and distortion coefficients are used to compensate I/Q input subjected to orthogonal demodulation of RF linearly and orthogonally modulate the input to be amplified is put to practical use in the terrestrial digital broadcasting.
Further, in non-patent literature 8, a terrestrial digital broadcasting transmitter manufactured as a product is described in which IM=−30 dB is realized in Doherty amplification of a carrier amplifier, a peak amplifier and a combined circuit, IM=−41 dB is realized in the distortion compensation and a complicated nonlinear filter is used to add historical (memory) distortion compensation and realize such low distortion and high efficiency as IM=−53 dB and power efficiency 27%.